1. Field of the Invention
The present invention relates to improved bipolar cells and gas depolarized rechargeable batteries, such as nickel hydrogen and zinc oxygen, containing such cells, and to methods for producing such batteries.
2. Discussion of the Prior Art
A requirement of bipolar batteries is the need for an electrolyte seal between individual cells to prevent short circuits. This requirement is complicated in batteries with a gas phase active material since communication between each cell and a common gas storage space must be provided. Thus both a liquid seal and a gas vent are required.
An additional problem with these batteries is that means must be found to prevent gas phase water transfer between cells either as water vapor or as a result of parasitic reactions which generate oxygen in the case of a nickel hydrogen battery or hydrogen in the case of a zinc oxygen battery. If the parasitic gases are transported to the bulk vapor space then they may preferentially recombine with the depolarizing gas in those cells closest to the gas space. For example, O.sub.2 generated during nickel electrode overcharge in a nickel hydrogen cell may react with hydrogen within the cell or be transported out of the cell and react in the gas storage vessel or in another electrode. O.sub.2 lost from the cell may be converted to H.sub.2 O in other cells or other parts of the battery. This leads to "dry out" and failure of the cell from which O.sub.2 is lost.
Bipolar cells and rechargeable batteries containing same are disclosed in commonly assigned U.S. application for patent, Ser. No. 08/626,992 filed Apr. 3, 1996, titled, "Bipolar Design for a Gas Depolarized Battery". A plurality of cells are suitably mounted within a pressure vessel, specifically within a central cylinder in a nested relationship. Each cell contains a metered predetermined quantity of electrolyte and includes a metallic bipolar cup having a base and an integral upstanding side wall encompassing the base. The upstanding side wall preferably is of truncated conical shape diverging with increased distance from the base, although other shapes may be utilized including the side wall being substantially coplanar with the base. An insulating material covers the upstanding side wall.
The metallic bipolar cup may be fabricated from a variety of materials including nickel, aluminum plated with nickel, stainless steel, metallic coated graphite composite and titanium. The side wall coating of insulating material is a hydrophobic fluorocarbon polymer such as TEFLON.RTM. (polytetrafluoroethylene).
The hydrophobic insulating coating plays a critical role in achieving optimum battery performance and life. This coating must be resistant to separation or delamination from the inside and outside walls of the metallic cup when exposed to the highly alkaline electrolyte solution and the severe electrochemical and thermal environment typical of the interior of a bipolar battery, particularly during operation in gravity-free outer space. This is essential in order to prevent electrolyte sharing between adjacent battery cells and to isolate or insulate adjacent cells electrically.
While TEFLON.RTM. represented a preferred hydrophobic insulating coating material, it delaminates from the walls of the cup after a relatively short period of time upon exposure to the strong alkaline electrolyte solution and severe electrochemical and thermal environment within the bipolar battery. Even when a primer coating is applied to the metal cup, prior to the application of the TEFLON.RTM. coating, the electrolyte attacks the interface of the metal and the primer and lifts the coating off.
Attempts to improve the bonding of the TEFLON.RTM. coating to the opposed surfaces of the annular outer cup wall by perforating the wall and heat-bonding opposed layers of the TEFLON.RTM. through the perforations were unsuccessful. The TEFLON.RTM. bonded tightly to itself, through the perforations, but did not bond to the metal in other areas. Also, a difference in the coefficient of expansion between the metal and the TEFLON.RTM. layer causes the TEFLON.RTM. layer to shrink tightly against the outer cup wall and to shrink away from the inner cup wall, causing distortion of the shape of the cup and leakage of the electrolyte.
It is the objective of the present invention to overcome the aforementioned disadvantages and limitations of the prior art by providing a hydrophobic insulation coating which has excellent affinity and retention properties for the metallic cell cups or dishes used in a bipolar battery. Such a coating must adhere to the metallic dish even after extended periods of exposure to the highly alkaline electrolyte and the severe electrochemical and thermal environment typical of that inside a spacecraft battery, without the need for a primer coating or perforations.